Problem: If $x \oplus y = x-7y$ and $x \boxdot y = 2x-6$, find $5 \boxdot (5 \oplus -4)$.
Explanation: We don't need to find $5 \oplus -4$ because $x \boxdot y$ depends only on the left operand. Find $5 \boxdot y$ $ 5 \boxdot y = (2)(5)-6$ $ \hphantom{5 \boxdot y} = 4$.